Which of the following statements correctly describes the ionizable groups in a peptide chain?

• A. The C-terminal has a free amino group, and the N-terminal has a free carboxyl group.
• B. Both the N-terminal and C-terminal have free amino groups.
• C. Both the N-terminal and C-terminal have free carboxyl groups.
• D. The N-terminal has a free amino group, and the C-terminal has a free carboxyl group.

Answer: (D)